How to find the longest line in a file using bash script?

Question

can anyone give a look to this bash script of mine please? I am trying to find the longest line in a file using a bash script so I wrote this.

#!/bin/bash

#Francesco Foschi length of a row in a file

let n=0
let max_length=0

while read row
do
    length=$(echo -n $row | wc -c)
    if[ ${#length} -gt ${#max_length} ]
    then
            let max_length=${#length}
    fi

    echo "$n row is $length charachters long"
    echo "$row"
    let n=n+1

done < $1

echo "longest line is $max_length charachters long"

exit 0

Every time I try to run the console says that I have a syntax error near the unexpected then token. What am I doing wrong??

BTW running of fedora28

I suggest you to check your code in https://www.shellcheck.net/ — ingroxd, Nov 24 '18 at 16:22
Bash is a supremely slow and tacky tool for this particular task. You want to investigate reimplementing the script in Awk for any real-world scenario. — tripleee, Nov 26 '18 at 05:56
I rolled back your latest edit. Questions on Stack Overflow should remain strictly questions; the way to mark something as a suitable answer is simply to accept an answer, like you already did. (The [question's edit history](/posts/53459879/revisions) still contains all the text if you want to recover it.) — tripleee, Nov 26 '18 at 05:57

score 5 · Answer 1 · answered Nov 24 '18 at 16:46

5

GNU wc has this functionality built in:

-L, --max-line-length
        print the maximum display width

answered Nov 24 '18 at 16:46

cody

11,045
3
21
36

I knew this but I am trying to go full POSIX – francesco foschi Nov 24 '18 at 17:00

score 2 · Accepted Answer · answered Nov 24 '18 at 16:35

try this:

#!/bin/bash
#Francesco Foschi length of a row in a file

let n=0
let max_length=0

while read row
do
    length=$(echo -n $row | wc -c)
    if [ ${length} -gt ${max_length} ]
    then
            let max_length=${length}
    fi

    echo "$n row is $length charachters long"
    echo "$row"
    let n=n+1

done < $1

echo "longest line is $max_length charachters long"

exit 0

score 2 · Answer 3 · answered Nov 24 '18 at 17:28

plain bash

#!/bin/bash
max=-1
while IFS= read -r line; do
    [[ ${#line} -gt $max ]] && max=${#line}
done < "$1"
echo "longest line is $max chars long"

This idiom is used to read the line exactly verbatim: IFS= read -r line

Demo:

create a file with leading/trailing whitespace and a backslash
```
$ echo ' h\bHello ' > file
```
this file is 10 bytes in size (not counting the trailing newline).

read it with plain read var

$ read line < file; printf %s "$line" | od -c
0000000   h   b   H   e   l   l   o
0000007

Only 7 chars: missing the backslash and the whitespace

add the -r option for read:
```
$ read -r line < file; printf %s "$line" | od -c
0000000   h   \   b   H   e   l   l   o
0000010
```
Now we have 8 characters (the "0000010" is octal), but still missing the whitespace.
add the IFS= variable assignment:
```
$ IFS= read -r line < file; printf %s "$line" | od -c
0000000       h   \   b   H   e   l   l   o    
0000012
```
10 characters (octal 12): now we have exactly what was written to the file in $line.

It's a pain to have to write IFS= read -r line all the time, but bash inflicts considerable pain on the programmer.

How to find the longest line in a file using bash script?

3 Answers3