Differences

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--- bash:files:read_a_file [2021/01/26 13:56] – [Field splitting, white-space trimming, and other input processing] peter
+++ bash:files:read_a_file [2021/01/26 14:19] (current) – [How to keep other commands from "eating" the input] peter
@@ Line 6: / Line 6: @@
 [[BASH:Files:Read a file:Basic read|Basic read]]
+[[BASH:Files:Read a file:Input source selection|Input source selection]]
 [[BASH:Files:Read a file:Read fields from a file|Read fields from a file]]
+[[BASH:Files:Read a file:Read from a command instead of a regular file|Read from a command instead of a regular file]]
 [[BASH:Files:Read a file:Read from an interactive shell|Read from an interactive shell]]
@@ Line 13: / Line 17: @@
 [[BASH:Files:Read a file:Skip Reading Comments|Skip Reading Comments]]
+[[BASH:Files:Read a file:Troubleshooting|Troubleshooting]]
-----
-====== Input source selection ======
-The redirection < "$file" tells the while loop to read from the file whose name is in the variable file.  If you would prefer to use a literal pathname instead of a variable, you may do that as well.  If your input source is the script's standard input, then you don't need any redirection at all.
-If your input source is the contents of a variable/parameter, bash can iterate over its lines using a here string:
-<code bash>
-while IFS= read -r line; do
-  printf '%s\n' "$line"
-done <<< "$var"
-</code>
 ----
-The same can be done in any Bourne-type shell by using a "here document" (although read -r is POSIX, not Bourne):
-<code bash>
-while IFS= read -r line; do
-  printf '%s\n' "$line"
-done <<EOF
-$var
-EOF
-</code>
-----
-====== Read from a command instead of a regular file ======
-<code bash>
-some command | while IFS= read -r line; do
-  printf '%s\n' "$line"
-done
-</code>
-----
-This method is especially useful for processing the output of find with a block of commands:
-<code bash>
-find . -type f -print0 | while IFS= read -r -d '' file; do
-    mv "$file" "${file// /_}"
-done
-</code>
-**NOTE:**  This reads one filename at a time from the find command and renames the file, replacing spaces with underscores.
-  * **-print0**:  uses NUL bytes as filename delimiters.
-  * **-d ''**:  Instructs it to read all text into the file variable until it finds a NUL byte.
-    * By default, **find** and **read** delimit their input with newlines; however, since filenames can potentially contain newlines themselves, this default behavior will split up those filenames at the newlines and cause the loop body to fail.
-    * **IFS= **:  Set to an empty string, because otherwise read would still strip leading and trailing whitespace.
-  * **|**:  Pipes the output from the find command into the while loop.
-    * This places the loop in a "sub shell", which means any state changes you make (changing variables, cd, opening and closing files, etc.) will be lost when the loop finishes.
-    * To avoid that, you may use a ProcessSubstitution:
-----
-<code bash>
-while IFS= read -r line; do
-  printf '%s\n' "$line"
-done < <(some command)
-</code>
-----
-===== My text files are broken!  They lack their final newlines! =====
-If there are some characters after the last line in the file (or to put it differently, if the last line is not terminated by a newline character), then read will read it but return false, leaving the broken partial line in the read variable(s).  You can process this after the loop:
-<code bash>
-# Emulate cat
-while IFS= read -r line; do
-  printf '%s\n' "$line"
-done < "$file"
-[[ -n $line ]] && printf %s "$line"
-</code>
-or:
-<code bash>
-# This does not work:
-printf 'line 1\ntruncated line 2' | while read -r line; do echo $line; done
-# This does not work either:
-printf 'line 1\ntruncated line 2' | while read -r line; do echo "$line"; done; [[ $line ]] && echo -n "$line"
-# This works:
-printf 'line 1\ntruncated line 2' | { while read -r line; do echo "$line"; done; [[ $line ]] && echo "$line"; }
-</code>
-The first example, beyond missing the after-loop test, is also missing quotes. See Quotes or Arguments for an explanation why. The Arguments page is an especially important read.
-For a discussion of why the second example above does not work as expected, see FAQ #24.
-----
-Alternatively, you can simply add a logical OR to the while test:
-<code bash>
-while IFS= read -r line || [[ -n $line ]]; do
-  printf '%s\n' "$line"
-done < "$file"
-printf 'line 1\ntruncated line 2' | while read -r line || [[ -n $line ]]; do echo "$line"; done
-</code>
-----
-===== How to keep other commands from "eating" the input =====
-Some commands greedily eat up all available data on standard input.  The examples above do not take precautions against such programs.  For example,
-<code bash>
-while read -r line; do
-  cat > ignoredfile
-  printf '%s\n' "$line"
-done < "$file"
-</code>
-will only print the contents of the first line, with the remaining contents going to "ignoredfile", as cat slurps up all available input.
-----
-One workaround is to use a numeric FileDescriptor rather than standard input:
-<code bash>
-# Bash
-while IFS= read -r -u 9 line; do
-  cat > ignoredfile
-  printf '%s\n' "$line"
-done 9< "$file"
-# Note that read -u is not portable to every shell. Use a redirect to ensure it works in any POSIX compliant shell:
-while IFS= read -r line <&9; do
-  cat > ignoredfile
-  printf '%s\n' "$line"
-done 9< "$file"
-</code>
-or:
-<code bash>
-exec 9< "$file"
-while IFS= read -r line <&9; do
-  cat > ignoredfile
-  printf '%s\n' "$line"
-done
-exec 9<&-
-</code>
-This example will wait for the user to type something into the file ignoredfile at each iteration instead of eating up the loop input.
-You might need this, for example, with mencoder which will accept user input if there is any, but will continue silently if there isn't.  Other commands that act this way include ssh and ffmpeg.  Additional workarounds for this are discussed in FAQ #89.